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SPARKS, Nev. — The 12-year-old student who opened fire on a Nevada middle school campus, wounding two classmates and killing a teacher before he turned the gun on himself, got the weapon from his home, authorities said Tuesday.

Washoe County School District police said they are still working to determine how the boy obtained the 9mm semiautomatic Ruger handgun used in the Monday morning spree at Sparks Middle School. The boy’s parents are cooperating with authorities and could face charges in the case, police said.

The violence started before the first bell of the day rang, as students filed off buses and gathered for class. The boy opened fire outside a school building, hitting one 12-year-old student in the shoulder. He then encountered Landsberry, 45. The teacher walked calmly toward the shooter and lifted his hands, asking the boy to hand over his weapon.

‘‘He was telling him to stop and put the gun down,’’ student Jose Cazares told NBC’s ‘‘Today’’ show Tuesday. ‘‘Then the kid, he yelled out ‘No!’ Like, he was yelling at him, and he shot him.’’

Still, his actions gave students enough time to run to safety, according to police.

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